As moritz said, this is actually a shell command. eval is both a shell and a Perl command, but in this case it is the shell version.

In most UNIX shells (not csh), $? is the status of the previous command (in Perl it is the exit status of the previous child process - not the same thing). In a shell, parentheses create a sub-shell which is a new shell environment (I use the term loosly) in the same process, so eval '(exit $?0)' in shell exits the sub-shell (not the current process). In shells, an evaluation of non-zero is considered false, and 0 is considered true (consider 0 to be 'success').

exec also is in both shells and Perl, and it replaces the current program with a new one in the same process, so it will replace the shell with perl.

In this case, the if 0 will be considered part of the command by a shell. However, in Perl the if 0 has precedence, so none of that will be executed if we are running perl, zero being false in perl (and most languages). Try:

When putting a smiley right before a closing parenthesis, do you:

Use two parentheses: (Like this: :) )
Use one parenthesis: (Like this: :)
Reverse direction of the smiley: (Like this: (: )
Use angle/square brackets instead of parentheses
Use C-style commenting to set the smiley off from the closing parenthesis
Make the smiley a dunce: (:>
I disapprove of emoticons
Other